Level of a liquid in a tank can be detected by many methods, viz., a float coupled to an external read-out either mechanically or magnetically, or devices that manipulate Ultrasound/Optical waves or capacitance based sensors. Each method of detection has its merits and demerits.
Cryogenic fluids such as liquid Nitrogen, liquid Oxygen, liquid Hydrogen and liquid Helium find extensive application in Industry, Aviation, Space Technology and Scientific Research. In most of these application areas, the study of the cryogenic liquids for their dynamic behavior, during filling or emptying of large cryogenic tanks, as well as, the reliable monitoring of these fluids quantitatively when such operations are effected at high flow rates has grown into an active area of research. In particular, the measurement of cryogenic fuel and the oxidizer levels in large tanks of the cryogenic systems is crucial task of paramount importance, particularly when the application area involves sensing and regulation of complex instrumentation tasks, in a coordinated manner.
A number of liquid-vapor (L-V) interface sensing methods were used to facilitate such level measurement. The known methods relied upon differences in the resistance, the capacitance, the acoustic impedance or viscous damping in order to sense the location of the L-V interface. Among these methods, only the capacitance based sensing systems received considerable attention for wide-spread application in aerospace industry. However, the factors related to high cost, slow response, heavy weight and potential ‘electric spark hazard’ pose several questions on their reliability, especially when used for the detection of the liquid levels in fuel tanks in aerospace industry.
To address the problems specific to level-detection in fuel tanks related to aerospace industry, a variety of fiber-optic systems have evolved. Such systems have many advantages over conventional level sensing approaches: for example, they are electrically passive in nature, ensure inherently spark-free level-sensing and involve no moving mechanical parts. In explosive environment characteristic to liquid Hydrogen and liquid Oxygen, such features help to realize a safe operation with increased efficiency and reduced maintenance load. Further, optical fibers are four times lighter per unit volume and six times stronger than copper wires. They neither produce nor are prone to EMI, thereby lifting the need for shielding/insulation, which makes them substantially lighter than their electric counterparts. Such weight reduction is crucial in aerospace applications. Owing to the increasing demand for these fiber optic systems in telecom industry, they are also becoming competitive in cost. Further, as these devices sense the liquid level through principles of reflection/refraction, they facilitate extremely high speed level-detection.
Well known fiber-optic systems rely upon devices like small total internally reflecting prisms or spherical lenses mounted at the ends of two optical fibers, or a conically shaped optical fiber tip or a U-shaped bent optical fiber, for level detection. Depending upon the application, the said devices may suffer from a few limitations. For instance, in such devices, intensity of the reflected light is the basis for level-sensing which is dependent upon the refractive indices of both the liquid as well as the material used for the prism. Since the latter changes with the ambient conditions, the said devices when specifically used for measuring the level of cryogenic liquids need individual calibration at each of the operating pressure and temperature. These devices are also frequently designed to operate at an angle of 180° deviation, which, in some sense, leads to relatively increased ‘volume’ of sensing device.
The fabrication of such sensing devices involves optically finishing at least three or more surfaces to a high degree of surface accuracy and maintaining a precise angle of the prism. Further, the position of the sensing device relative to the light guiding devices is invariably fixed and not allowed any freedom for linear or angular motion.
U.S. Pat. No. 6,801,678 discloses a fiber optic level detector using Fresnel reflection based liquid sensor in which part of the transmitted light reflects back at interface boundaries where the refractive indices are different and the reflected light intensity depends upon the refractive index of the fiber and liquid under ambient conditions.
The refractive characteristics of solid prisms do not permit transmission of optical beams without deviation. The emerging optical beam always deviates from the incident optical beam, whether the prism is in air or immersed in liquid. Further, when immersed in liquids, the amount of deviation in such prisms depends upon the refractive index of the liquid medium as well as that of the material used for fabrication of the prism. Both these factors necessitate recalibration and repositioning procedures in solid prism based fiber optic level detectors whenever the liquid under level measurement is changed or its operating conditions change dynamically.